1. Field of the Invention
The present invention relates to electrodes for electrochemical reactions in acid electrolytes and more in particular to carbon-base electrodes.
There are innumerable processes wherein it is usefull or convenient to reduce or oxidize a compound present in ionic form (dissolved) in an electrolyte. Most often the electrolyte is an acid aqueous solution containing ions of the dissolved compound to be reduced or oxidized.
A most typical processing of this type is represented by a so-called redox flow cell, the development of which has received a decisive thrust as a potentially efficient and simple way of storing excess or recoverable electrical energy in chemical form (secondary battery systems).
Of course this is not the only area of utility of electrochemical redox processing, many chemical synthesis, regeneration of pickling liquors and pollution control processes have the necessity of reducing or oxidizing certain dissolvable compounds.
In all these type of electrochemical redox processes, the half-cell conditions at one or at both electrodes must prevent undesired parasitic oxidation or reduction reactions in order to ensure a high yield (and therefore a high energy efficiency) of the specific oxidation or reduction reaction to be performed at the particular electrode (half-cell). Most typical is the requirement of preventing electrolysis of the solvent. In an aqueous electrolyte it is essential to prevent water electrolysis and this may require that either oxygen evolution at the positive electrode and/or hydrogen evolution at the negative electrode be effectively prevented.
These requirements tend to exclude the use as electrodes in such half-cell systems of conductive materials having an intrinsically low overvoltage for the unwanted reaction. In the case of an aqueous electrolyte, this will exclude materials exhibiting a particularly low oxygen and/or hydrogen overvoltage.
These requisites, coupled to the requirement for the electrode to be perfectly resistant to aggressive acid electrolytes and to the ionic species intervening (nascent species) in the half-cell reaction, greatly restrict the number of usable materials.
One of the electrodic material that is more widely used under these peculiar conditions remains carbon, in its various forms.
2. Discussion of Related Art
Graphite, amorphous carbon, activated charcoal, glassy carbon, are the basic electrically conductive materials used for fabricating electrodes for this type of applications.
Solid graphite or the mechanically stronger glassy carbon plate are often used as bipolar electrodes. However, their electrodic performance is rather low.
Considerations on the rate limiting factors of the half-cell redox reactions generally dictate the use of electrodes having a large active surface for unit of projected cell (electrode) area in order to be able to support the half-cell reaction at an acceptable rate without causing an abrupt increase of the half cell voltage due to intervening &lt;&lt;saturation&gt;&gt; effects of the mass transfer mechanisms to and from active sites on the electrode surface, eventually across the so-called electrode's double layer, often compounded in empirically determined potential coefficients. An increase of the half-cell voltage will in turn promote parasitic half-cell reactions, for example oxygen and/or hydrogen evolution at the respective positive and negative electrodes.
Porous carbon electrodes, in the form of a Teflon.RTM. (a registered trademark of E. Du Pont de Nemours) bonded porous layer of carbon particles directly bonded to a ion exchange membrane or microporous separator of the electrochemical cell have been proposed for redox flow batteries (re: GB-A-2,030,349-A), however these bonded electrodes structures though ensuring a truly minimized cell gap, the thickness of which may correspond to the thickness of the ion exchange membrane or microporous diaphragm used as the cell separator (so-called SPE cells from the acronym of Solid Polymer Electrolyte), pose a serious often insuperable problem of efficiently and reliably collecting and distributing electric current to and from the bonded electrode layer. The limited electrical conductivity of these resin bonded porous carbon particle layers and the practical difficulty of establishing reliable point-like contacts between a suitable current distributor and the electrodically active bonded layer by pressure make this cell architecture impracticable especially in case of relatively large area cells, stacked together in a bipolar cell assembly that may include up to one hundred or even several hundreds of cells in electrical series.
The use of porous carbon fabrics or felts sandwiched between the cell separator and a current collector, in lieu of bonded carbon layers, though ensuring a good lateral conductivity of the porous electrode layer, represented by the carbon fabric or felt, still presents problems as far as the establishment of reliable pressure-held electrical contacts with the current distributor structure is concerned, especially in multicell stacks.
In general, pressure held electrical contacts between carbon materials in electrolytes show extremely erratic contact resistances that tend to increase in time, probably due to the build up of filming compounds chemically bonded to the carbon material.
In an attempt to overcome the problems of reliably distributing and drawing current from a porous (three-dimensional) active electrode structure and of ensuring a low-resistive path for the electric current to and from the potentially active sites of a relatively porous electrode structure (three-dimensional), it has been proposed to directly bond an adequately porous (high specific surface), three-dimensional carbon electrode structure, permeable to the electrolyte, to a suitable electrically conductive substrate which may constitute the end wall of the electrode (or half-cell) compartment or a fluid impervious bipolar septum that provides for electrical continuity between a positive electrode bonded to one face and a negative electrode bonded to the other face thereof, belonging to two distinct cells, respectively, of a stack of cells.
The bipolar electrode structure so constituted hydraulically separates the negative half-cell compartment of a cell from the positive half-cell compartment of an adjacent cell in the stack or battery of cells in electrical series with one another.
The electrically conductive separating septum may be of a suitable thermoplastic resin, for example high density polyethylene (HDPE) mixed with a styrene-ethane/butyl-styrene (SEBS) block polymers or with a styrene-isoprene-styrene copolymer loaded with carbon black graphite fibers and/or carbon powder or other corrosion resistant conductive material powder in order to provide for an adequate electrical conductivity.
Alternatively, attempts have been made to bond a carbon felt on the face of a solid glassy carbon, graphite or carbon plate with carbon loaded conductive adhesives. However, these attempts were frustrated by an insufficiently reliable bonding and for large cell areas the use of solid graphite or glassy carbon plate is costly and prone to disaster cracks.
Known bipolar electrode structures, when adapted for a specific use in redox flow cells, have been found to have serious drawbacks.
Even the lamination process of a highly porous and permeable carbon electrode layers to a thermoplastic aggregate is extremely difficult because of the inability of applying a lamination pressure capable on one side to promote an intimate adhesion of the felt or fabric to the partly fluidized thermoplastic aggregate without permanently collapsing the felt or fabric and/or embedding it in the thermoplastic aggregate. Moreover, the requisites of such a post-lamination process for joining together and in an electrical path continuity condition the thermoplastic aggregate to the porous electrode structures contrast with the requirements of a low resistivity of the aggregate, by severely limiting in practice the amount of conductive powder that can be loaded without impairing the possibility of post-laminating the porous electrodes onto the pre-formed conductive sheet.
Another intrinsic limitation of these hot pressed composites is that often practically only relatively few filaments or fibers of the felt or fabric electrode becomes bonded in an electrically conductive manner to the electrically conductive thermoplastic aggregate sheet.
Distribution of electric current through the remaining porous layer of the felt or fabric relies on fortuitous electric paths among microscopically distant points of the porous structure of the felt or fabric. Most of these fortuitous electric paths through the mass of the porous electrode structure entail fibers that are oriented substantially parallel to the plane of the composite and which form or contribute to form relatively tortuous and long paths which inevitably represent highly resistive electric paths.
Whichever the arrangement used, another severe drawback of the so hot pressed carbon fabric or felts of carbon fibers is represented by their limited residual &lt;&lt;permeability&gt;&gt; to a streaming electrolyte being flown through the electrode compartment.
Indeed, though the fabric or felt may be rendered quite hydrophilic by appropriate treatments and be readily permeable to the liquid electrolyte, their intertwined structure represents a relatively large pressure-drop path for a streaming electrolyte being pumped through the half-cell compartment.
On the other hand, the intertwined structure cannot be to loose or have an unlimitedly large void ratio because electrical bulk conductivity through the felt may decline intolerably. Therefore, the streaming electrolyte inevitably will tend to flow almost exclusively through preferential &lt;&lt;by-pass&gt;&gt; paths, typically through flow spaces or channels defined by the current distributor structure and/or through gaps that may form between the surface of the ion exchange membrane or microporous separator and the fabric or felt electrode.
In practice the electrolyte within the intricate mass of carbon fibers or filaments of the felt or fabric electrode will be &lt;&lt;refreshed&gt;&gt; practically only through local diffusion processes, driven by intervening concentration gradients rather than being more effectively and uniformly &lt;&lt;refreshed&gt;&gt; by the hydraulic flow imposed by pumping.
Indeed, the overvoltage developed at the half-cell upon an increase of the current density may in large measure be due to a grossly inadequate mechanical transport (distribution) of the reacting species toward the population of active sites within the three-dimensional electrode structure.
All these aspects of known carbon electrode structures and the intrinsic critical aspects and limitations thereof document the great difficulty of realizing an electrodic structure of low electrical resistance and capable of sustaining high current densities of up to 1000 A/m.sup.2 or even higher without a severe decline of the voltage characteristics of the electrochemical redox cell.
Finally the known bipolar electrode assemblies are rather heavy, the main contribution to their weight being represented by the conductive thermoplastic septum or backbone.